Which pointing device has a ball on its top that can be rolled with fingers and the internal rollers sense the motion that is transmitted to the computer?

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So how does an object's visual angle affect our perception of its size? First, if the visual angle of an object is too small we will be unable to perceive it at all. Visual acuity is the ability of a person to perceive fine detail. A number of measurements have been established to test visual acuity, most of which are included in standard eye tests. For example, a person with normal vision can detect a single line if it has a visual angle of 0.5 seconds of arc. Spaces between lines can be detected at 30 seconds to 1 minute of visual arc. These represent the limits of human visual acuity.

The mouse has become a major component of the majority of personal computer systems and general-purpose workstations sold today, and is the little box with the tail connecting it to the machine in our basic computer system picture (Figure 2.1). It is a small, palm-sized box housing a weighted ball -- as the box is moved over the tabletop, the ball is rolled by the table and so rotates inside the housing. This rotation is detected by small rollers that are in contact with the ball, and these adjust the values of potentiometers. The changing values of these potentiometers can be directly related to changes in position of the ball. The potentiometers are aligned in different directions so that they can detect both horizontal and vertical motion. The relative motion information is passed to the computer via a wire attached to the box, and moves a pointer on the screen, called the cursor. The whole arrangement tends to look rodent-like, with the box acting as the body and the wire as the tail; hence the term 'mouse'. In addition to detecting motion, the mouse has typically one, two or three buttons on top. These are used to indicate selection or to initiate action. Single-button mice tend to have similar functionality to multi-button mice, and achieve this by instituting different operations for a single and a double button click. A 'double-click' is when the button is pressed twice in rapid succession. Multi-button mice tend to allocate one operation to each particular button.

The trackball is a little like an upside-down mouse. The weighted ball is rotated inside a static housing, the motion being detected in the same way as for a mechanical mouse, and the relative motion of the ball moves the cursor. Because of this, the trackball requires no additional space in which to operate, and is therefore a very compact device. It is an indirect device, and requires separate buttons for selection. It is fairly accurate, but is difficult to draw with, as long movements are difficult. Trackballs now appear in a wide variety of sizes, the most usual size of ball being about the same as a golf ball, with a number of larger and smaller devices available. Some of the smaller devices are becoming increasing popular for use with notebook and portable computers, with more enlightened designs having them actually built in, either above or below the keyboard, rather than clipping on to the side of the machine. The size and 'feel' of the trackball itself affords significant differences in the usability of the device: its weight, rolling resistance and texture all contribute to the overall effect.

The sonic tablet is similar to the above but requires no special surface. An ultrasonic pulse is emitted by a special pen which is detected by two or more microphones which then triangulate the pen position. This device can be adapted to provide 3D input, if required.

One of the mainstays of the new virtual reality systems (see Chapter 15), the dataglove is a 3D input device. Consisting of a Lycra glove with optical fibers laid along the fingers, it detects the joint angles of the fingers and thumb. As the fingers are bent, the fiber optic cable bends too; increasing bend causes more light to leak from the fiber, and the reduction in intensity is detected by the glove and related to the degree of bend in the joint. Attached to the top of the glove are two sensors which use ultrasound to determine 3D positional information as well as the angle of roll, that is the degree of wrist rotation. Such rich multi-dimensional input is currently a solution in search of a problem, in that most of the applications in use do not require such a comprehensive form of data input, whilst those that do cannot afford it. However, the availability of cheaper versions of the dataglove ($50 rather than the $10,000 required for the full dataglove) will encourage the development of more complex systems that are able to utilize the full power of the dataglove as an input device. There are a number of potential uses for this technology to assist disabled people, but cost remains the limiting factor at present.

There are occasions when modal dialog boxes are necessary, for example when a major fault has been detected, or for certain kinds of instructional software. However, the general philosophy of modern systems suggests that one should minimize the use of pre-emptive elements, allowing the user maximum flexibility.

Adaptivity is automatic customization of the user interface by the system. Decisions for adaptation can be based on user expertise or observed repetition of certain task sequences. The distinction between adaptivity and adaptability is that the user plays an explicit role in adaptability, whereas his role in an adaptive interface is more implicit. A system can be trained to recognize the behaviour of an expert or novice and accordingly adjust its dialog control or help system automatically to match the needs of the current user. This is in contrast with a system which would require the user to classify himself as novice or expert at the beginning of a session. We discuss adaptive systems further in Chapter 12. Automatic macro construction is a form of programming by example, combining adaptability with adaptivity in a simple and useful way. Repetitive tasks can be detected by observing user behaviour and macros can be automatically (or with user consent) constructed from this observation to perform repetitive tasks automatically.

It is interesting to note that, if Alison were a novice user, she would be more likely to check her actions and thus notice the mistake -- an unnoticed button miss is an expert slip. As all but the most extensive user testing of a new device must, by definition, be with novices, there is no way this would be detected -- which is perhaps why most on-screen buttons have this problem. We hope this demonstrates how, on occasions, semi-formal hand analysis may even be more effective than real user testing.

Control flow is centralized in the notifier, which relieves the application program of much of the tedium of processing every possible event passed to it by the window system. But this freedom from control does not come without a price. Suppose, for example, that the application program wanted to produce a pre-emptive dialog box, perhaps because it has detected an error and wants to obtain confirmation from the user before proceeding. The pre-emptive dialog effectively discards all subsequent user actions except for ones that it requires, say selection by the user inside a certain region of the screen. To do this in the read--evaluation paradigm is fairly straightforward. Suppose the error condition occurred during the processing of an event of type type_2. Once the error condition is recognized, the application then begins another read--evaluation loop contained within that branch of the case statement. Within that loop, all non-relevant events can be received and discarded. The pseudocode example given earlier would be modified in the following way:

repeat     

     read-event(myevent)

     case     myevent.type

          type_1 :     

               do type_1 processing

          type_2 :     

               Š

               if     (error-condition) then

                    repeat     

                         read-event(myevent2)

                         case     myevent2.type

                              type_1 :


                              .

                              .

                              .

                              type_n :

                         end case

                    until (end-condition2)

               end if

               Š

          .

          .

          .

          type_n :     

               do type_n processing

     end case

until (end-condition)

Reconfigure The server must detect that the client has failed and reconfigure the rest of the system accordingly. The client's failure can be detected by standard network failure codes, or by timing out the client if it is silent for too long. Reconfiguring will involve resetting internal data structures, and informing other participants that one of them is unavailable and why. Do not let them think their colleague is just being rude and not replying!

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Is a stationary pointing device with a ball on its top or side to move the pointer using a trackball you rotate the ball with your thumb fingers or the palm of your hand?

What is the ball mouse?

Which is the ball that can be rotated with the fingers or palm of the hand?

What input device translates motions of a ball rolled on a flat surface to the monitor screen?